DESCRIPTION (Applicant's Abstract): The distinguishing neuropathological feature of Alzheimer's disease (AD) is the presence of amyloid-containing neuritic plaques in the patient's brain. Activated microglia, the CNS-specific macrophage cells, are a prominent feature of these plaques where they function to release reactive free oxygen radicals, reactive nitrogen species, cytokines and proteases which may contribute to plaque generation. Recent evidence indicates that plaque density and the prevalence of Alzheimer's disease is positively associated with the APOE4 genotype. Apolipoprotein-E (apoE) , the protein component of lipoproteins, promotes the transport of lipids and cholesterol required for cell survival. Our preliminary data indicate that apoE may have another action in promoting the production of nitric oxide (NO) by microglia. In the cardiovascular system, apoprotein- containing LDL (low density lipoproteins) induced NO production and as a consequence reduced macrophage-mediated LDL oxidation. Our finding that apoE promotes NO production may signal a similar role for NO in the CNS. Our preliminary data also indicate that the 4 kDa amyloid-beta peptides (A beta 1-40 and A beta 1-42) alters microglial function by stimulating superoxide production. Thus, Dr, Colton proposes that A beta may disrupt the regulatory effect of apoE on NO production shifting the superoxide/NO balance in favor of oxidative stress. Using primary cultures of microglia from neonatal hamster CNS, a human clonal microglial cell line and human monocyte derived macrophages, Dr Colton will examine the effect of apoE on resting and immune-activated cells. Both superoxide and NO production will be measured in response to the common 2, 3, and 4 isotypes of apoE protein and pre-oxidized forms of apoE. Interaction of a po E with membrane receptors will be studied. Interference of this regulatory pathway by A beta alone, apoE alone or A beta plus apoE will be tested since AB forms stable complexes with apoE. The possible involvement of superoxide reacting with NO to form peroxynitrite and the role of A beta/apoE complexes in A beta-mediated dysregulation will be explored. Finally, the consequences of this regulatory pathway will be examined by determining the level of lipoprotein oxidation and cell survival in the presence and absence of apoE alone, AB alone or A beta/apoE complexes.